1. Field of the Invention
The invention relates to the sealing of pores in porous rigid articles, particularly metal articles such as metal castings and sintered metal products but also materials such as wood or inorganic materials such as brick, stone, or concrete. It is however important that the pores in the material are not so big as to permit leaching out of the impregnant. In a modification of the invention, the impregnant of the invention is used to impregnate materials such as paper and cloth.
2. Description of the Prior Art
The impregnation of metal castings with resins to seal porosity is a process which is well established. Porosity in castings is invariably inconsistent in size and shape, thus rendering the components unusable due to leakage. This is especially true where they are subjected to pressure. Apart from this well known cause for rejection of castings used in the motor industry there are many other problems caused by porosity including plating failures, "blow out" during stove enamelling, and sites for corrosion, entrapment of organic material and possible bacteria growth.
Generally, porosity can be divided into three types: through porosity, blind porosity and enclosed porosity. Through porosity causes leakage and is the type with which founders are primarily concerned. Blind porosity, having one entrance to the surface only, will not produce a leakage but can cause surface finishing problems through absorption of treatment fluids. Enclosed porosity causes no problems unless present in excess where it can cause structural failure. Other similar defects often encountered in the foundry include cold laps, cracks, blow holes, and inclusions all of which are often referred to, incorrectly, as forms of porosity. Frequently examination of a casting rejected for leakage and marked by the inspector as porous, reveals that it actually has a cold lap, crack or a blow hole.
Various method of sealing porosity employed over the years include plugging, coating with epoxy resin, and welding. All of these techniques are highly labour intensive and therefore expensive, and there is no guarantee that the treatment will be successful.
A straightforward method in common use employs a solution of a high molecular weight polymer. The cleaned components are simply dipped into the solution for several minutes and on removal and subsequent evaporation of the solvent sealing is accomplished. This technique is not suitable for use on highly machined components that are to meet tight dimensional tolerances due to the thin film of polymer left on the surfaces. In addition any surface treatments such as conversion coating, anodic or chemical, that are called for, must be carried out prior to dipping. As penetration is limited and incomplete filling of the pores with resin is an inescapable consequence of employing solvent, more than one treatment may be required and tightness of the casting at high pressures must not be expected.
The process of vacuum impregnation came into use some twenty five years again and over this period many types of sealants have been used, all with varying degrees of success, from "Bakelite".TM. varnish to sodium silicate and a wide range of polyesters. Modern processes of this type employ specially tailored unsaturated polyesters that have high penetration power in combination with low viscosity monomers such as styrene. Typically, the unsaturated polyesters are reaction products of phthalic anhydride and maleic anhydride with propylene glycol. A combination of inhibitors and catalysts is chosen to stabilise the viscosity of the impregnant during production runs and to give suitable curing at temperatures in the region of 130.degree. C. Cross linking of the linear polyester by styrene results in a hard, solvent resistant resin which completely fills the pores. A typical manufacturing process involves placing the cleaned, cold castings in an autoclave and subjecting them to a vacuum of not less than 12.7 mms of Hg for a minimum of 20 minutes. At this point the impregnant is admitted to the autoclave and brought to a level approximately two inches above the castings. Pressure of 90 to 100 lbs. per sq. in. is then applied to the autoclave for 30 minutes or more.
The impregnant fluid is then returned to the storage tank to allow the removal of the castings and their transfer to the wash tank for the removal of the surface film left by the sealant. After a short draining period, the parts are submerged in an oil for 45-60 minutes at 130.degree. C. or alternatively placed in a hot air recirculation oven for 11/2 hours. The final steps are to transfer the parts to a fresh water rinse to remove surface contamination. With this existing method of treatment pressure tightness of up to 12,000 lb. per sq. in. can be obtained within a temperature range of -40.degree. C. to +250.degree. C.
Other systems include anaerobically curing impregnants e.g. of the type described in U.K. Patent Specification No. 1,297,103.
The present invention has been particularly developed for use with impregnants of the type described in U.K. Patent Specification No. 1,547,801 but is not limited thereto. Such impregnants cure thermally at a temperature of about 90.degree. C. and are thus conveniently cured by contacting the casting with hot water, either in the form of a bath or in the form of a spray.
Impregnation processes previously used have been batch processes but these are not altogether convenient e.g. in the treatment of cylinder blocks for motor vehicles. It is however possible to treat metal castings individually. This generally necessitates blocking off of the casting and pressurising with impregnant from within, or drawing a vacuum and submerging the casting in the impregnant. Neither of these two ways is highly desirable because of the inherent problems of sealing up the openings in the casting.